In the art of microphones, a coil caused to vibrate in the vicinity of a magnet has long served as the means for generating an electrical signal that analogues the acoustical vibrations to be picked up. In traditional contact microphones, the coil is attached to a diaphram at the end of an eustuchian tube whose other end is attached to the surface to be miked. Such devices work well but are rather fragile and expensive.
In more recent contact microphones coils and magnets have been for the most part replaced by piezo-electric elements which when stressed, produce an electrical output which varies depending on the stress, thus producing an A.C. signal output that analogues the vibrations of the resonating member to which the microphone is attached. This type of microphone has the advantages of being very simple and fairly small and light. A disadvantage of this type of microphone however is that since the output is rather small and the low frequency response is generally weak, a pre-amp of some description is almost always required for boosting the signal and for equalization. Such pre-amps are an added expense, are bulky and require a power source. Another disadvantage of piezo electric micophones is that due to the vagueries of the piezo electric elements a "flat" response giving a signal which accurately coresponds to the vibrations of the miked surface is difficult to obtain. Yet another disadvantage of this type of microphone is that in actual practice arranging the microphone in such a manner as to be stressed adequately to produce a strong signal is difficult. Sometimes increasing the pressure engaging the mike to the surface increases the output of the mike however this results in the problem the vibrating characteristics of the surface become altered due to the extra force.